Late to the party but for what it's worth I've also got a 318 and when I had heads redone used valves, seats and guides appropriate for unleaded so don't use any additives.
I try to use super unleaded whenever I'm filling up but aren't too fussy about which brand.
Was in France recently and a few times had to use what was available (95 )- no obvious ill effects (no pinking etc).
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